This invention relates to the manufacture of printed circuit boards and, more particularly, to a process for treating the surfaces of interconnecting through-holes in printed circuit boards to prepare the surfaces for subsequent metallization.
It is now commonplace in the art to provide printed circuit boards having more than one plane or layer of circuitry, wherein the layers of circuitry are separated one from the other by an insulating substrate material, i.e., a dielectric. In the simplest version of such printed circuit boards, an insulating substrate material is provided on both sides with conductive circuit patterns. In other versions, which have become enormously popular over the past several years, a so-called multilayer circuit board is provided. These multilayer boards are comprised of one or more innerlayers of conductive circuitry in addition to layers of circuitry on the outer-facing surfaces of the board, with insulating substrate material separating each layer or innerlayer of circuitry.
In order to provide conductive interconnection between or among the circuitry layers and/or innerlayers in printed circuit boards, one or more through-holes are formed in the board (i.e., normal to the plane of the board) and the wall surfaces of the through-holes are then metallized.
Since the metallized through-holes provide the means for conductive interconnection between or among circuitry layers and/or innerlayers, the quality of the metal layer provided on the through-hole surfaces is extremely important. In particular, it is necessary that the metal deposit adhere tightly to the through-hole surfaces and that the metal deposit be in the form of an essentially continuous, void-free, layer. While to a degree these quality parameters are dependent upon the nature of the metal depositing solution employed and the conditions under which metallization is carried out, to a more significant degree these parameters are dependent upon the receptivity of the through-hole surfaces to acceptance of the metal layer.
Generally speaking, through-hole metallization is accomplished through use of electroless metal depositing solutions which require that the surfaces to be coated with the metal be catalyzed to promote the electroless deposition. Numerous techniques and proposals exist in the art for treating or conditioning through-hole surfaces so as to enhance their receptivity to the deposit of catalytic species and to the subsequently-applied electroless metal deposit so as to arrive at an adherent, void-free metal coating effective to provide the desired conductive interconnection between or among circuitry layers on or in the board.
Multilayer printed circuit boards pose unique problems in through-hole metallization. In the formation of through-holes (e.g., by drilling) in these boards, the edge portions of metal innerlayers which are exposed at the hole surfaces can become "smeared" with the dielectric material. Such smeared innerlayer surfaces at the through-hole exhibit poor receptivity to catalyst and deposited metal and lead to lack of adherence and/or incomplete coverage with respect to the metal deposit. Accordingly, it is known in the art to "desmear" through-hole surfaces to remove resinous smear from circuitry innerlayers at the through-hole surfaces, such as by vapor honing, secondary drilling, plasma etching or, more commonly, by the action of chemicals such as sulfuric acid, chromic acid or alkaline (e.g., sodium, potassium) permanganate. In the course of desmearing processes, particularly when chemical means are employed, the process also is known to "etch back" the dielectric material at the hole surfaces, thereby exposing more of the metal circuitry innerlayer surface at the through hole surfaces and aiding in subsequent adhesion of catalyst and deposited metal.
The utilization of permanganate to desmear and/or etch-back and/or clean through-holes so as to condition the through-hole surfaces for subsequent metallization has received considerable attention in the art. British Patent Specification No. 1,479,556 describes a process for removing resin smear from printed circuit board through-holes and from holes made in wire assemblies by contacting the hole surfaces with an alkaline permanganate solution having a pH of 13-14 at a temperature between 35.degree. and 70.degree. C., preferably also containing a wetting agent of the fluorocarbon variety.
U.S. Pat. No. 4,425,380 to Nuzzi, et al. also relates to the desmearing of through-holes in printed circuit boards and in wire assemblies, employing a process involving contact of the hole surfaces with an alkaline permaganate solution, followed by contact with compounds effective to remove manganese residues from the surfaces so as to avoid interference of these species with the subsequent electroless depositing step.
In permanganate-based desmear/etch-back processes for through-holes formed in circuit boards wherein the insulating substrate material is epoxy (e.g., glass-reinforced epoxy resin), the art also has taught that the efficiency or effectiveness of the permanganate treatment generally can be enhanced by first treating the through-hole surfaces with a solvent for the epoxy to open up or soften or swell the epoxy resin structure. See, e.g., Kukanskis, "Improved Smear Removal Process For Multilayer Circuit Boards", IPC Technical Paper No. 435 (Oct., 1982); F. Tomaivolo, et al., "Alkaline Permanganate Treatment In Etch-Back Processes," Trans IMF, 1986, 64, 80; U.S. Pat. No. 4,515,829 to Deckert, et al.; U.S. Pat. No. 4,592,852 to Courduvelis, et al.; and published PCT patent application No. WO 85/05755 to DelGobbo, et al. In this same vein, there is described in the art a method for post-desmear treatment of through-holes to better prepare them for subsequent metallization, in which the through-hole, after being desmeared, is treated with a solvent for the substrate resin, followed by treatment with an alkaline permanganate solution. See, e.g., U.S. Pat. No. 4,597,988 to Kukanskis, et al.
The epoxy solvents described in pre-permanganate treatment processes of the foregoing references are of the water-soluble type, a choice dictated by the desirability of using these solvents in aqueous solution and by the desirability of facilitating rinsing by insuring that the solvents will be miscible with rinsing water. It has been found in accordance with the present invention that, notwithstanding the desirability of employing water-soluble solvents, the effectiveness of alkaline permanganate treatment in cleaning and/or desmearing and/or etch back and/or post-desmear conditioning processes for through-holes, particularly those in boards where the dielectric is epoxy-based or contains epoxy, is generally enhanced through contact of the through-hole surfaces, prior to the permanganate treatment, with water-immiscible organic liquids which serve to penetrate and/or soften and/or swell and/or otherwise act upon the dielectric material.
According to another aspect of the invention, the water-immiscible organic liquid is used in association with water and, optionally, other water-soluble compounds, in order to enhance removal of the water-immiscible organic liquid by subsequent water rinsing and, importantly, to provide a pre-permanganate treatment which will be optimally effective irrespective of the composition of the dielectric, for example, whether it be epoxy-based or a polyimide or other thermosetting or thermoplastic material, or the like.